1. Field of the Invention
The present invention relates to an ignition timing adjusting system for a spark-ignition internal combustion engine, and more particularly, to a system for realizing an advance of the spark or ignition timing in relation to an altitude (that is, a height above sea level) at which the engine is operating. Said system is also called an "altitude compensating system" herein.
It is well-known that the ignition timing of a spark-ignition engine should be varied according to different operating conditions in order to ensure the most effective and smoothest operation of the engine. In an engine for automobiles, for example, the normal ignition timing is generally preset so that an effective and smooth operation can be achieved when the engine is idling and warm. Accordingly, at higher speeds, under a partial-load (or part-throttle) operating condition, or when the engine is cold, the ignition timing should be advanced.
Further, it is well-known that when the engine is operating at a relatively higher altitude the ignition timing should be considerably advanced, as compared with the case when the engine is operating at a low altitude, because the air is rarer at higher altitudes. The normal ignition timing of an engine for automobiles, for example, is generally selected so as to be proper for engine operation at low altitudes, because it is there that automobiles are normally used. Sometimes, however, automobiles are used at higher altitudes with the ignition timing being present for the engine operation at low altitudes, since conventional engines are provided with no system for adjusting the ignition timing in relation to the engine operating altitude. If the engine is operating at a higher altitude than that for which the ignition timing was set, a lag in ignition results in a decrease in the output power, thus making engine response and performance poor. In addition, such operation results in an increase in the quantity of harmful constituents contained in the exhaust gas. If the engine is operating at a lower altitude than that for which the ignition timing was set, pre-ignition will bring about early breakdown of engine parts. In addition, such operation also results in an increase in the quantity of harmful constituents contained in the exhaust gas.
2. Description of the Prior Art
There are many types of known ignition timing adjusting systems. One of the most well-known is a vacuum advance mechanism which comprises a vacuum actuator, linked to a rotatable breaker plate of the spark distributor and which vacuum actuator responds to a vacuum produced in the intake tube so as to rotate said breaker plate, thereby advancing the ignition timing. However, while a conventional vacuum advance mechanism can realize the ignition timing advance in relation to the load condition of the engine, it can not do so in relation to the engine operating altitude.
Another well-known type of system is an electrical advance mechanism. This comprises two or more sets of breaker points incorporated in the distributor and an electrical switching device for selecting any one of said sets of breaker points, thereby providing a proper ignition timing advance. This mechanism may be adapted for adjusting the ignition timing in relation to the engine operating altitude, by controlling said switching device in relation to the engine operating altitude. However, this system has disadvantages. The electrical switching elements are very expensive and service and maintenance thereof are hard to obtain because adjusting or repairing of two or more sets of breaker points is very difficult and time consuming.